Embodiments of the invention are described below with reference to hammer drills. However, the inventive concept is not limited to such devices and a clutch embodying the invention can equally find use in other power tools such as circular saws or grinders, for instance.
During normal operation of a hammer drill a drill bit is caused to rotate whilst, at the same time, the drill bit receives repeated impacts. Such drills are often used for working masonry, or the like. The impact mechanism can be of a pneumatic or percussion type, known in the art. Furthermore, the hammer drill can be arranged to operate in hammer-drill mode, where the drill bit receives impacts as it rotates, and drill mode, where the impact mechanism is disengaged resulting in the drill bit rotating without receiving impacts. Often, hammer drill tools are provided with a hammer-only mode, for use with chisel tools, where the tool receives a series of impacts but does not rotate.
It is desirable to provide a torque overload clutch mechanisms in a rotary hammer drill which can interrupt drive from the drill's motor when a torque force exceeding a predetermined torque is applied to a tool bit. Such a clutch mechanism is particularly desirable for relatively powerful hammer drills. The clutch and associated predetermined torque should be arranged such that the drill operates normally as the drill bit engages a work-piece. However, if the drill bit becomes blocked or jammed in the work-piece then a torque force exceeding the predetermined torque is applied to the clutch, causing the clutch to operate or ratchet and interrupt power from the motor to the hammer drill output. If this situation occurs in a relatively powerful hammer drill which is not provided with such a clutch, then the motor would continue to apply power to the drill bit causing the hammer drill body to rotate relative to the work-piece. Furthermore, in a device which operates without a clutch, the motor might stall, or components of the drive train or gearbox might become dislocated causing damage or high levels of wear to themselves or other mechanisms.
A clutch mechanism for a pneumatic hammer drills has been proposed in WO2004/024398. In the arrangement described, the clutch is arranged on a driven gear, between the motor's gear and a bevel pinion which is arranged to mesh with a spindle's drive gear.
DE2522446 describes a handheld power tool which comprises an intermediate shaft with two fixed drive gears having different parameters. The gears cooperate with a spindle gear arrangement consisting of a large diameter gear wheel and a small diameter gear wheel. The small diameter gear wheel is press fitted on to a cylindrical projection of the large diameter gear wheel which is disposed on an inner sleeve element such that it can move with respect to the sleeve and so that an output spindle is formed. The output spindle is non-rotatably, but axially displaceably mounted on the tool spindle by means of the sleeve element. A ring element comprising clutch teeth is fixedly secured to the rear end of the sleeve and positioned within a recess formed by the inner circumferential surface the rear section of the gear wheel an end face of which comprises clutch teeth arranged to cooperate with the clutch teeth of the ring element. The clutch teeth are urged into engagement by springs disposed on the other end of the sleeve to the clutch teeth.